This invention relates to steam turbines and, more particularly, to a method for optimizing for different exhaust pressures and different levels of mass flow without different size final stage turbine blades.
The traditional approach to meeting the needs of the electric utilities over the years was to build larger units requiring increased exhaust annulus area with successive annulus area increases of about 25%. In this way, a new design with a single double flow exhaust configuration would be offered instead of an older design having the same total exhaust annulus area but with two double flow LP turbines. The newer design would have superior performance in comparison to the old design because of technological advances.
In recent years, the market has emphasized replacement blading on operating units to extend life, to obtain the benefits of improved thermal performance (both output and heat rate), and to improve reliability and correction of equipment degradation. In addition, the present market requires upgraded versions of currently available designs with improved reliability, lower heat rate and increased flexibility. If the new designs were retrofittable on the older counterparts as well as being the optimum configurations for the diversity of applications, substantial economies could be achieved in both engineering and manufacturing resources.
The latter stages of the steam turbine, because of their length, produce the largest proportion of the total turbine work and therefore have the greatest potential for improved heat rate. The last turbine stage operates at variable pressure ratio and consequently this stage design is extremely complex. Only the first turbine stage, if it is a partial-arc admission design, experiences a comparable variation in operating conditions. In addition to the last stage, the upstream low pressure (LP) turbine stages can also experience variations on operating conditions because of (1) differences in rated load end loading, (2) differences in site design exhaust pressure and deviations from the design values, (3) hood performance differences on various turbine frames, (4) LP inlet steam conditions resulting from cycle steam conditions and cycle variations, (5) location of extraction points, (6) operating load profile (base load versus cycling) and (7) zoned or multi-pressure condenser applications versus unzoned or single pressure condenser applications.
While all but the lowest pressure feedwater heater extraction flow vary linearly with and in direct proportion to unit throttle flow, the lowest pressure heater extraction flow varies at a greater rate than the throttle flow and also varies in response to changes in condenser pressure. This produces changes in inlet angle to the downstream stage and to a lesser extent affects the performance of the stage that immediately preceded this extraction point.
Since the last few stages in the turbine are tuned, tapered, twisted blades with more selective inlet angles, the seven factors identified above have greater influence on stage performance.
FIG. 1 illustrates the effect of end loading in the inlet angle to the last stage stationary blade of an exemplary steam turbine. This graph plots "incidence" on the vertical axis against blade height on the horizontal axis for two different values of end loading, one at 6000 lb/hr/ft.sup.2 (=29280 kg/hr/m.sup.2) and the other at 11500 lb/hr/ft.sup.2 (=56120 kg/hr/m.sup.2). The dashed lines represent predicted values while the shaded areas represent ranges of measured values. Incidence is the difference between the blade and fluid angles at inlet. Note that while the incidence angle varies about the predicted design angle at full load, the incidence angle deviates from the predicted angle at partial load. Similar changes in inlet angle but of lesser magnitude were identified on the next upstream stator blade.
There are many variations in extraction arrangements and standard blade gagings for steam turbines. Many of the differences between the L-2 stator blade gagings relate to non-reheat versus reheat applications. Furthermore, single flow elements of triple flow LP frames have different extraction arrangements but the same blading as the double flow element. In the triple flow systems, only one of the two flow paths (single flow or double flow) can be matched from the standpoint of incidence.
If a double flow LP turbine were operating at optimum efficiency at a given exhaust pressure with a single pressure condenser and if the condenser were converted to a two zone multi-pressure condenser with the same surface area, the pressure at one end of the double flow element would increase while the pressure at the other end would decrease. Neither end would be operating at optimum efficiency although there would be an improvement in heat rate because the average condenser pressure would be lower with the zoned condenser. The end with the lower exhaust pressure needs more flow area while the end with the higher exhaust pressure needs less flow area. Prior studies have demonstrated that the total exhaust area for the optimum zoned condenser application is about the same or slightly smaller than the total flow area of the unzoned arrangement. The conventional approach to optimizing such a system would be to select different size last row blades in each half of the zoned double flow LP element. This would result in a greater proliferation of blade sizes to achieve optimum performance.